The ceramics industry provides materials having properties that are critical for many applications. In W. D. Kingery et al. in Introduction to Ceramics, John Wiley & Sons, New York, 1976, page 3, "ceramics" is defined as the art and science of making and using solid articles which have as their essential component, and are composed in large part of, inorganic materials such as pottery, porcelain, refractories, structural clay products, abrasives, porcelain enamels, cements, glass, nonmetallic magnetic materials, ferroelectrics, manufactured single crystals, and a variety of other products.
In contrast to metal objects which are tough (where "toughness" is defined in D. W. Richerson, Modern Ceramic Engineering, Properties, Processing, and Use in Design, Marcel Dekker, Inc., New York, 1982, page 136, as having high capability to survive impact, the application of a structural load at a very high loading rate usually to a localized area) and which yield and deform, ceramic objects are hard and brittle and much less tough, fracture with little or no deformation, and require fabrication techniques which differ considerably from those for making metal objects. One technique for fabricating ceramic objects that is widely used in the glass-making industry involves melting the ceramic and shaping it as it cools and solidifies. A second general technique used to make ceramic objects involves compacting a finely-ground ceramic powder into the ceramic body. Prior to compaction and heating, a binder may optionally be added to the powder. This results in a plasticized compact that may be handled and machined more easily.
Much research in the ceramic industry targets the improvement of the physical properties of ceramic materials for use in applications where both material hardness and toughness are required. These efforts have led to the development of composite materials known as cermets. Cermets are made from ceramic materials and metals, and consist of a porous ceramic matrix having metal occupying the pores of the matrix. Cermets retain the hardness properties of the ceramic from which they are composed, and are also able to withstand greater impact stresses due to the metal network within the cermet.
Various techniques for fabricating cermets are known in the art. In U.S. Pat. No. 5,019,539 by Clarr et al. entitled "Process for Preparing Self-Supporting Bodies Having Controlled Porosity and Graded Properties and Products Thereby," issued May 28, 1991, a method for making cermets where a ceramic powder and a metal powder are combined and pressed into a compact which is then heated above the melting point of the metal is described. In example 1 of the '539 patent, column 9, lines 22-58 a is detailed procedure for the formation of a cermet of boron carbide (B.sub.4 C) and zirconium (Zr) is illustrated. Powders of B.sub.4 C and of Zr are combined, shaken, placed in a graphite crucible, and pressed with a die to make a pre-form. The die/pre-form assembly is heated under argon in a furnace, and the final product is a B.sub.4 C /Zr composite body having porosity as shown in FIGS. 1-4 of Clarr et al. In the '539 patent, a second method for making cermets is also described, where an initial ceramic/metal compact formed by first combining powdered metal and powdered ceramic and pressing, is then contacted with molten metal which infiltrates the compact to form a cermet. Both methods described in the '539 patent include the initial mixing of ceramic powder and metal powder to form cermets.
Alternatively, ceramic powder may first be compacted, sintered into a hard, porous ceramic matrix, and finally infiltrated by molten metal to form a cermet. The sintering process involves heating the compact in order to promote strong bonds between the individual ceramic particles while densifying through the removal of pores within the body. It is the sintering process which transforms the fragile and easily disfigured ceramic body resulting from initial compaction of ceramic powder into a strong, useful ceramic component.
In Modern Ceramic Engineering Properties, Processing and Use in Design by D. W. Richerson, Marcel Dekker Inc., 1982, the sintering (i.e. densification) techniques used in fabricating ceramic matrices are reviewed (see chapter 7, pages 217-259). Not all ceramic materials may successfully be densified by sintering. The sintering process may be successful if mass transport within the compact can by induced by heating the compact. The starting ceramic compact, known in the art as the "green body," is formed by pressing a ceramic powder into a shape, or by adding a plasticizing binder to the powder and then pressing. Additionally, an additive known in the art as a "sintering aid" or "densification aid" may be also be added to the ceramic powder prior to pressing. The sintering aid may be added in the form of a sintering aid precursor which is transformed into the true sintering aid. The sintering aid or sintering aid precursor may also enhance the structural integrity of the green body as a binder would. However, a binder is added solely to produce a more machinable green body and then removed prior to sintering the green body by heating the green body above the boiling temperature of the binder, but below the sintering temperature of the ceramic. By contrast, a sintering aid facilitates the densification of the green body into the final hard ceramic object during sintering, and remains within final hard ceramic body after sintering.
In Richerson, page 243, a table which includes a list of ceramic materials and their appropriate sintering aids is displayed. The following pairs taken from the table illustrate several known combinations of ceramic/sintering aids, respectively: alumina (Al.sub.2 O.sub.3) with lithium fluoride (LiF), magnesium oxide (MgO) with LiF or sodium fluoride (NaF), beryllium oxide (BeO) with lithium oxide (Li.sub.2 O), silicon nitride (Si.sub.3 N.sub.4) with magnesium oxide (MgO) or yttrium oxide (Y.sub.2 O.sub.5) or beryllium silicon nitride (BeSiN.sub.2), silicon carbide (SiC) with boron (B) or alumina or aluminum (Al). Several of the above combinations ceramic materials, such as MgO or Si.sub.3 N.sub.4, may be sintered with different sintering aids. The choice of sintering aid for a particular ceramic is determined experimentally. Thus, one particular material which acts as a sintering aid for a particular ceramic material may or may not act as a sintering aid for a different ceramic material.
The addition of sintering aids may allow sintering of a ceramic which would otherwise sinter only under extremely high pressures, or perhaps not all. For example, SIC, Si.sub.3 N.sub.4, B.sub.4 C sinter only under extremely high pressures, if at all. In Ceramics for High Performance Applications by S. Prochazka, edited by J. J. Burke et al., Brook Hill Publishing Co., 1974, pages 239-252, it is disclosed that the densification of powdered SiC is extremely difficult in the absence of a sintering aid, and diamond-forming conditions are needed to form objects having near theoretical density. However, with the addition of 1% boron as a sintering aid, SiC may be sintered at pressures as low as 100 PSI.
Sintering aids may be added directly to the ceramic powder, or may be formed from sintering aid precursors which are added to the ceramic and which transform into sintering aids upon heating. For example, in "Microstructure and Densification of Sintered (B+C)-Doped .beta.-Silicon Carbide" by W. Braue et al., Materials Research Society Symposium Proceedings, 237, edited by A. R. Barron et al., Materials Research Society, Pittsburgh, Pa., 1994, page 271, the addition of a phenolic resin to SiC and heating of the resin/SiC mixture is described. The phenolic resin is a sintering aid precursor, and heating of the SiC/resin above the sintering temperature decomposes the resin into carbon which acts as the true sintering aid.
It has been shown that hard, porous ceramic bodies are made by sintering a ceramic powder compact, and that infiltration of the sintered compact by metals yields cermets having the hardness of a ceramic and enhanced toughness due to the internal metal network. It may be advantageous to control the porosity of either the ceramic green body prior to sintering, or the hard compact which results from sintering the green body. The controlled porosity desired may be, in particular, a graded porosity where the porosity gradually increases from one part of the ceramic body to another. Infiltration of metal within such a graded-porosity ceramic body yields a cermet which may have graded physical properties resulting from the internally graded structure of the cermet.
The advantages of graded-porosity cermets and procedures for making them have been described in the art. One general procedure for forming graded-porosity cermets involves initial mixing of ceramic and metal powder followed by pressing and sintering to form the graded-porosity cermet. Another general procedure involves infiltration of a metal into a graded-porosity ceramic matrix. Several procedures for making ceramic matrices having a controlled porosity and, in particular, a graded porosity, are described below.
In U.S. Pat. No. 3,868,267 by G. E. Gazza et al. entitled "Method of Making Gradient Ceramic-Metal Material," which issued on Feb. 25, 1975, a method for making cermets having graded properties, whereby a graded-porosity boron-containing ceramic compact such as compacted boron carbide (B.sub.4 C) is infiltrated by silicon and aluminum is disclosed. In the '267 patent, the graded-porosity ceramic is made by first placing ceramic powder into a mold and vibrationally redistributing the powder particles by their appropriate particle sizes within the mold, then cold pressing the powder to form a porous ceramic compact, and finally infiltrating each separate volume of the compact with molten material. Further, details of the procedure for the fabrication of a porous ceramic body are found in column 2, lines 12-24, and include first separating ceramic powder into 2 particle size fractions, placing the first fraction having smaller particle size into a cold press die and tapping the die, placing the fraction having the larger particle size in the die above the first fraction, tapping the die again, and cold-pressing the fractions together to form a porous compact. This compact is then infiltrated with other substances such as silicon and aluminum to form a graded material. Although the '267 patent teaches the formation of a graded-porosity ceramic compact, there is no teaching of the use of a sintering aid precursor or a sintering aid.
In U.S. Pat. No. 5,525,374 by M. A. Ritland et al. entitled "Method for making Ceramic-Metal Gradient Composites," which issued on Jun. 11, 1996, a process for producing graded porosity ceramic bodies which are then infiltrated to produce cermets is described (see column 9, lines 25-46). First, a free-standing, sintered ceramic body having a porosity gradient is fabricated. Then, the ceramic body is infiltrated with metal to produce a cermet having graded properties. The '374 patent describes various methods for controlling the porosity in the green body, which are summarized herein. In one method, the ceramic powder may be subjected to vapor-phase sintering to form a porous green body having a controlled porosity. In a second method, the porosity of a green body may be controlled by forming agglomerates of the ceramic powder. For example, as described in column 6, lines 18-29, calcination of 50 micron size aluminum hydroxide (Al(OH).sub.3) particles produces alumina (Al.sub.2 O.sub.3) agglomerates which may be subsequently used to form a green body having a chosen porosity. A third method involves varying the pressure during compact formation to aid in controlling the total porosity of the green body. In a fourth method, the green body may be pre-sintered at a temperature slightly below the sintering temperature of the ceramic to control the final porosity of the final sintered body. Another method which may control the pore size and pore distribution includes varying the total sintering time.
In the '374 patent, column 8, lines 49-53, it is stated that sintering the ceramic without the addition of a sintering aid may produce a sintered ceramic body having open porosity. In the '374 patent, column 9 lines 32-34, it is also stated that a ceramic body having a homogeneous particle distribution can be sintered in a temperature gradient to produce a ceramic body having varying levels of porosity. Although the '374 patent describes the use of a sintering aid or sintering aid precursor to densify the ceramic body during sintering, the sintering aid is mixed directly with the ceramic powder prior to sintering. The '374 patent does not teach the addition of a sintering aid or sintering aid precursor to ceramic powder or a green body where a specific mode of addition is responsible for producing the graded porosity in the ceramic body.
In U.S. Pat. No. 5,098,870 by Clarr et al. entitled "Process for Preparing Self-Supporting Bodies Having Controlled Porosity and Graded Properties and Products Produced Thereby," which issued on Mar. 24, 1992, a method for producing self-supporting bodies comprising borides or boride/carbide mixtures having controlled porosity and graded properties is described. In the '870 patent, column 6, lines 54-61, it is stated that the pore sizes within the compacts are due to the metal within the compact, and that larger metal particles within the compact are equivalent to larger pore sizes. In the '870 patent, column 6, line 67 through column 7, line 8, a process for making a B.sub.4 C/metal body having a relatively dense first surface and relatively porous second surface opposing the first surface is described. This process involves mixing relatively large metal particles with B.sub.4 C powder near the surface of the body chosen to be the porous side, and mixing relatively small metal particles with the B.sub.4 C powder near the surface chosen to be the more dense side.
In the '870 patent, column 7, lines 31-50, a method for making a cermet by combining ceramic powder with molten metal is described. The metal in the cermet is equivalent to the porosity within the cermet, where a higher concentration of metal translates into a higher porosity. The porosity produced when a body or pool of metal is used in lieu of powdered metal may not be as controlled or uniform as the porosity produced when powdered metal is used.
In the '870 patent, column 7, lines 51-66, a process for forming a porous B.sub.4 C body is also described, wherein B.sub.4 C powder is combined with various combustible additives such as gelatin, corn starch, and wax, which permit the formation of an initially dense pre-form and aid in the porosity-forming process. These additives leave pores when they are removed prior to sintering the green body and allow further control of porosity within the final, hard porous ceramic object.
Prior teachings have shown that cermets are an important class of materials which can be made by either direct mixing of metal and ceramic powder, or by infiltration of a metal into a porous ceramic body. Methods which allow easy access to preformed graded-porosity ceramic bodies are desirable because these bodies may be infiltrated to form cermets having graded properties. Known methods involve the arrangement of particle sizes to achieve the desired gradient.
Therefore, an object of this invention is to provide a process for making ceramic bodies having a graded porosity from a ceramic powder having a monomodal particle-size distribution.
Another object of this invention is to provide a process for making cermets having a graded properties.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.